1. Field of the Invention
This invention concerns the technical field of lignin separation. In particular the present invention relates to a method for lignin separation from spent cooking liquor, called black liquor. Further the invention relates to a lignin product obtainable by the above mentioned method and use of said product.
2. Description of Related Art
In a modern, energy-optimized pulp mill, there is a surplus of internally generated energy. With today's modern processes, bark can be exported while the remaining energy surplus, in the form of mixtures comprising other burnable residues, is burned in the recovery boiler, with a relatively low efficiency, especially with regard to electricity production. There is also often a problem because the heat transfer capacity in the recovery boiler is a narrow sector, a so called bottleneck, which limits the production of pulp in the mill. The recovery boiler is further the most expensive (instrument) unit in the pulp mill.
To separate lignin from black liquor is an interesting solution to these problems. In this way, the energy surplus can be withdrawn from the process in the form of a solid biofuel and can be exported to e.g. a power station, where the fuel can be used more efficiently than in the recovery boiler of the pulp mill. This lignin is also a valuable material for production of “green chemicals”. A further alternative to energy production is to use the extracted lignin as chemical feedstock. Further, lignin extraction leaves a black liquor for combustion with a lower thermal value, which in turn leads to a lower load on the recovery boiler. This gives in a short term perspective possibilities for increased pulp production. In the long term perspective lower instrument costs for the recovery boiler are expected.
There are several possible procedures for such a separation, and industrial applications have been known for a long time. Already in 1944, Tomlinson and Tomlinson Jr were granted a patent for improvements to such a method. The separation method used today is to acidify the black liquor so that the lignin is precipitated in the form of a salt. The solid phase is separated from the liquor and can thereafter be cleaned or modified. There are industrial applications in operation today where lignin is separated from black liquor for use as special chemicals. One example of such a process is the precipitation of lignin from black liquor by acidification with carbon dioxide. The suspension is taken to a storage vessel for conditioning of the precipitate after which the solid lignin is separated and washed (with acidic wash water) on a band filter, and is finally processed to the desired state.
However, if the separated lignin is to be used for fuel the demands on cleanliness and properties are completely different from those when the application is for use as a special chemical. A successful washing of the precipitated lignin is very important, to obtain a lignin fuel with a reasonably low ash content and a low tendency to cause corrosion and to be able to return as much as possible of the cooking chemicals to the chemicals recovery unit. Also important is to minimize filtering resistance in order to minimize filtering area as well as promote possibilities to reach a high dry solid content for the lignin production.
In laboratory studies of such a separation mentioned above (which also is found in our experimental part) the result was in some cases a “pure” lignin (sufficiently clean for qualified fuel usage), but relatively large problems arose through blockage of the filter cake. The flow of wash water was reduced to almost zero in some tests. In other tests, an uneven washing of the filter cake occurred with high concentrations of inorganic substances (primarily sodium) in the lignin as a result. These problems could be reduced, as was found during the course of the experiment, by washing with highly acidic washing water (pH=1) in order to obtain the quickest possible reduction of the pH in the filter cake. On an industrial scale, however, such a procedure leads to a very high consumption of acid and accordingly a such procedure is very inefficient.
Accordingly, there is a need for a method where lignin can be separated using small amounts of acid whereby an essentially pure lignin product is obtained which can e.g. be used as fuel or for the production of chemicals. Further it would be desirable that said method achieves a lignin product suitable for use as fuel with reasonably low ash content and a low tendency to cause corrosion.